1. Field of the Invention
The present invention relates to a combustion device of the forced intake and discharge type for introducing combustion air into a burner and discharging combustion gases from the burner with a fan, and more particularly to a closure detector for detecting an unwanted closure of an intake or discharge tube of the combustion device prior to abnormal combustion.
2. Description of the Related Art
Combustion devices of the forced intake and discharge type, which are widely used as FF (forced flue)-type air heaters, for example, introduce combustion air from the outdoors and discharge combustion gases with a fan (combustion fan). Therefore, these combustion devices are capable of burning a fuel to heat rooms without contaminating the indoor air.
In order to detect an unexpected closure such as a clogging of the intake or discharge tube of a combustion device, the combustion device may have a closure detector which comprises, for example, an orifice disposed in the intake tube and a pressure sensor for detecting the value of a pressure difference between the air pressure upstream of the orifice and the air pressure downstream of the orifice.
When an unwanted closure of the intake tube occurs, the intensity of an air flow passing through the orifice while the fan is in operation is reduced, resulting in a reduction in the pressure difference between the air pressure upstream of the orifice and the air pressure down-stream of the orifice. Therefore, while the fan is in operation, the value of the pressure difference detected by the pressure sensor is monitored, and when the detected pressure value becomes smaller than a predetermined threshold value, it is judged that the intake tube is undesirably closed, and the burner can be turned off.
However, the value of the pressure difference detected by the pressure sensor may suffer an error because of an individual characteristic difference or aging of a pressure detecting element, such as a thin-film semiconductor device, of the pressure sensor. If the detected value suffers such an error, then an unwanted closure of the intake tube cannot accurately be detected by comparison between the value of the pressure difference detected by the pressure sensor and the predetermined threshold value.